Faster than light communication

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Discussion Overview

The discussion centers around the concept of faster-than-light (FTL) communication, specifically through the hypothetical scenario of rotating a long cylinder to transmit information instantly from one end to the other. Participants explore the implications of rigidity and the transmission of forces within materials, as well as comparisons to other thought experiments.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant proposes a method of FTL communication using a rotating cylinder, suggesting that rotation at one end would cause instantaneous rotation at the other end.
  • Another participant challenges this idea, stating that the rotation would not be instantaneous due to the time it takes for forces to propagate through the material, which is slower than the speed of light.
  • A third participant notes that nothing is perfectly rigid, implying that real-world materials would not behave as the original thought experiment suggests.
  • One participant compares the scenario to a thought experiment involving a flashlight, arguing that while the light beam may appear to sweep at superluminal speeds, it behaves differently than anticipated.
  • Another participant uses the analogy of a jello cylinder to illustrate that twisting one end would create a slow-moving torsion wave, further emphasizing the lack of perfect rigidity in materials.
  • A reference to a FAQ on the topic of pushing a "rigid" rod is provided, indicating that twisting is conceptually similar but does not resolve the issue of FTL communication.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views regarding the feasibility of FTL communication through the proposed method. The discussion remains unresolved with ongoing debate about the nature of rigidity and force transmission.

Contextual Notes

Limitations include assumptions about material rigidity, the nature of force propagation, and the applicability of thought experiments to real-world physics. These aspects remain unresolved within the discussion.

Ian75
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Imagine a small cylinder with a dot at the top. Rotating the cylinder 1 degree clockwise of 0 degrees could signify a 1; and 1 degree anticlockwise of 0 degrees a zero. 0 degrees would be a space.

Someone rotating one end of the cylinder would cause the other end of the cylinder to rotate instantly. Now imagine the cylinder extended to the length of one light year. Rotating one end of the cylinder would cause the other end to rotate instantly, yet a signal sent from the same end and traveling at the speed of light would take one year to arrive. This would be faster than light communication - or am I missing something really obvious?
 
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Ian75 said:
This would be faster than light communication - or am I missing something really obvious?
Yes. This (flagged by me):
Ian75 said:
Rotating one end of the cylinder would cause the other end to rotate instantly
Instantly is wrong. It takes time to transport the force from one end to the other end and this is even slower than the speed of light. The cylinder is only kept in one piece by electromagnetic forces on molecule level. So an action on one end has to be transported via this chain to the other end.
 
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Nothing is perfectly rigid.
 
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This is very similar to the thought experiment of taking a flashlight and sweeping it. Far enough away, the argument goes, the light beam would sweep with superluminal speed. But, what actually happens is that the beam of light behaves more like a waterhose at that point.
 
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Ian75 said:
Someone rotating one end of the cylinder would cause the other end of the cylinder to rotate instantly.
Think of a cylinder made of jello. If you rotate one end then you will set up a torsion wave that moves so slow that you can see it.

Steel isn't perfectly rigid, and on relativistic it would be like jello.
 
This thread is attracting FTL posts which are violations of PF rules, so I'm going to go ahead and close it.

Thread locked.
 

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